Creates a JSON search API using custom feed_* hook

What we want to achieve :

With those modifications we want to create a plugin doing a search API (ie. allowing ajax search requests), outputting JSON. The feed_* hook seems appropriate to do it but we were missing the exported variables.

Those modifications include :

* Make the regular 'doView' search and feed use the same code. Doing this, we have access in custom feeds to exported variables (like pagination)
* Add a hook "after_search" to be able to modify search results before doView or custom feed output. For example, we have a user_attributes plugin (age, gender, work activity) and for now we export those custom user attributes (for each item owner) in the search before_html hook. With that after_search hook, we can do it there and not in the before_html anymore and make those available in our custom JSON feed.

I'm open to suggestions if you got better ideas but it's a working solution ;-).
Merge commit 'pr/1574' into 1574

Conflicts:
	oc-includes/osclass/controller/search.php
Merge pull request #1577 from conejoninja/1574

1574
Admin Dashboard > Settings > General (small UI fix proposal)

This is a small fix for custom date & time formats, when user enters custom date or time, js preview is breaked into 2 lines which looks bad. Simple expansion of the table.table-small width and/or adding &lt;br&gt; is not going to fix it properly (right table then skips one raw when preview pops-out), thus I've dropped the table and replaced it with divs.

* margin was added to simulate old table aliment, not sure which one really looks better, probably adding one more time option for symmetry would be best. 

* I was not sure where to put style in what less file, but most probable candidate is page-settings.less.

* Although .custom-date-time div wrapper has no style, it was added for code completeness and clarity.

* div class="help-box" style="clear:both; float:none;" has a small very important inline code to fix following help box, I was too lazy to define additional class or id just for it (it is not used anywhere else).

CSS
>.custom-date-time{ }
>.custom-date-time #date{ float:left; width:150px; }
>.custom-date-time #time{ margin-top:20px; float:left; width:150px; }
Admin Dashboard > Settings > General (small UI fix proposal)

addition to #1579
Merge pull request #1579 from dev101-osclass/patch-1

Admin Dashboard > Settings > General (small UI fix proposal)
Merge pull request #1580 from dev101-osclass/patch-2

Admin Dashboard > Settings > General (small UI fix proposal)